Paper mill core structure for improved winding and support of paper mill roll

ABSTRACT

Paper mill cores of the invention are structured to allow winding and unwinding chucks to distort the interior diameter and shape of the interior body wall of the core while resisting distortion of the core exterior to prevent center burst failure. The paper mill cores of the invention include a multi-ply zone of high strength, but relatively compliant, paperboard plies within the outer 70% (based on the total body wall thickness) of the body wall having a thickness of at least about 4 mm. An interior zone constituting at least about 25% of the total thickness of the body wall is formed from extremely high strength, extremely high density paperboard plies. The overall wall thickness of the core is preferably at least about 15 mm, and is thus preferably increased, as compared to the wall thickness of conventional high strength wide paper mill cores formed entirely of extremely high strength, extremely high density, non-compliant paperboard plies.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. application Ser. No.09/411,522, filed Oct. 4, 1999, the entirety of which is herebyincorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention is directed to a paperboard core structure forwinding and supporting heavy rolls of wide, continuous paper sheet. Moreparticularly, the invention is directed to a multilayer paperboard corestructure having a high flat crush strength and dynamic strength forwinding and supporting rolls of continuous paper mill sheet having awidth exceeding 100 in. (254 cm.), and a roll diameter typically aboveabout 50 in. (127 cm.).

BACKGROUND OF THE INVENTION

[0003] Wide, heavy paperboard cores for supporting wide and heavy papermill rolls used in high end applications such as gravure printing, areconstructed to meet a demanding set of strength requirements including ahigh flat-crush strength and a high dynamic strength. These and otherstrength requirements are necessary because the paperboard cores aresupported internally at their ends by expanded chucks during winding andunwinding operations in which the roll of paper wound onto the core hasa weight above two tons (1,800 kg), typically approaching or exceedingfive tons (4,500 kg.), and spans a width typically between about 100inches and 140 inches (254 cm. and 356 cm.). These cores are currentlysupplied in two standard internal diameter (ID) sizes of 3 in. (76.2mm.) and 6 in. (152.0 mm) in the United States, and in a standard ID of150.4 mm in Europe, (which corresponds to the 6 in. ID U.S. paper millcore).

[0004] The problem known as “center burst” failure of these heavy andwide paper rolls has frustrated the paper mill industry for years. Inparticular, paper rolls that appear perfect when wound, subsequentlyfail for no apparent reason in the unwinding operation during printing.The symptoms of center burst failure are well known and consistent.Paper in the portion of the roll near the core, and at the ends of theroll above the unwind chucks, bursts, and patches can be squeezedsideways out of the roll. These patches significantly increase thechances of a web break during the printing process resulting in costlydowntime at the press room. Also, patches can lay on top of theremaining sheet causing misprints and quality problems. Such defectivepaper rolls returned to the paper mill supplier can have a major impacton profitability.

[0005] The exact cause of center burst failure has not been identified.For example, examination of paperboard cores supporting paper mill rollshaving center burst failure defects has not resulted in identificationof any correlated defects in the cores. Center burst failure occurs inabout 1-8% of paper mill rolls and it is known that the frequency ofcenter burst failure has been increasing. This increase has beenattributed to the use of wider and heavier paper rolls. Also, ascompared to fine quality papers of prior decades, current fine qualitypapers are often thinner with lower friction surfaces and often havelower strength due to a higher recycled fiber content. As a result, itis much more difficult to maintain winding tension and to build auniform structure throughout the diameter of the paper roll. Theseproblems have been addressed by modifications of paper mill windingapparatus to provide continuous monitoring and control of paper tensionduring winding along with the application of torque in such a way as togive a desired profile of winding tension. These modifications buildfriction and compression into the wound paper mill roll in order tominimize possible harm to the paper roll due to slipping of variouslayers on the roll during shipping and subsequent unwinding.Nevertheless, center burst failure persists and continues to increasewith no predictable pattern or cause.

[0006] The assignee of the present application has developed variouspaperboard core structures and techniques to address specific problemswith paperboard cores designed for specific end uses. For example, U.S.Pat. No. 5,393,582 issued Feb. 28, 1995 to Yiming Wang, Monica McCarthy,Terry D. Gerhardt, and Charles G. Johnson discloses paperboard tubestructures of enhanced flat crush strength. These structures involve theuse of zones or layers of paperboard plies distributed within the tubewall such that lower strength, lower density paperboard plies arepositioned on the exterior and interior portions of the tube wall, whilehigher density, higher strength paperboard plies are positioned in themiddle or central portion of the paperboard tube wall. U.S. Pat. No.5,505,395 issued Apr. 9, 1996 to Yanping Qiu and Terry D. Gerhardtdiscloses paperboard tube structures that address tube inside diameterdeformation problems which arise when tubes are supported on a mandreland subjected to substantial radial compression loading as a result ofhighly retractive yams or films wound onto the core under high tension.These core structures involve the use of zones of high strength, highdensity paperboard plies positioned on the exterior and interiorportions of the tube wall with lower strength, lower density paperboardplies positioned in the middle or central portion of the paperboardwall. In addition, factors influencing radial crush strength ofpaperboard tubes are discussed in T. D. Gerhardt, External PressureLoading of Spiral Tape Paper Tubes: Theory and Experiment, Journal ofEngineering Materials and Technology, Vol. 112, pp. 144-150 (1990).

[0007] Although these and other tube structures and modifications havebeen proposed for dealing with specific paperboard core end userequirements, the center burst problem associated with the wide heavypaperboard cores for wide heavy paper mill sheet rolls has not beenshown to be caused by any property or apparent defect in paperboardcores. In addition, the extreme weight of paper mill paper rolls and theextreme dynamic stresses applied to the paperboard core during windingoperations dictate that these paperboard cores must exhibit high flatcrush and dynamic strength, thus limiting the range of coremodifications available to address possible core structure variantswithin the desired wall thickness range to decrease center burstfailure. Possible core modifications are further limited in that theinside diameter portions of the paperboard core are formed fromextremely high strength, high density paperboard plies because of theso-called chuck “chew-out” forces applied by the surface of the windingchuck to the inside surface of the core during winding. Indeed, becauseof these various requirements, paperboard cores for wide, heavy papermill rolls are conventionally constructed entirely of extremely highstrength, high density paperboard plies. Typically, the paperboard plieshave a density exceeding 0.80 g/cc and a sufficient number of plies areused in the case of a 6 inc. (150 and 152 mm) internal diameterpaperboard core, to provide a wall thickness of about 13 mm (0.512inch), or in the case of a three inch inside diameter paperboard core, awall thickness of about 16 mm (0.630 inches).

[0008] More recently, the assignee of the present application hasmodified the traditional paper mill winding core constructions toincorporate the optimized flat crush strength constructions disclosed inU.S. Pat. No. 5,393,582 issued Feb. 28, 1995 to Yiming Wang, MonicaMcCarthy, Terry D. Gerhardt, and Charles G. Johnson. Accordingly, thecurrent paper mill winding core constructions of assignee employ zonesof paperboard plies at the exterior and interior portions of the tubewall in which the ply density is within the lower portion of the highstrength range (0.80 to 0.92 g/cc), together with a central zone ofplies having a density in the upper portion of the high strength range(0.80 to 0.92 g/cc). The center burst performance of these coresgenerally exceeds, or is at least comparable to, the center burstperformance of conventional, competitive paper mill winding cores.

[0009] Nevertheless, despite the high strength and high durability ofthe conventional paperboard core structures, and of Assignee's modifiedcore constructions; and even though winding apparatus has been modifiedto optimize the build of paper mill rolls, occurrences of center burstfailure persist and have been increasing.

SUMMARY OF THE INVENTION

[0010] The invention provides paperboard core structures that cansubstantially reduce or eliminate center burst defects in wide, heavypaper mill rolls. The paperboard core structures of the invention arebased on identification of a previously unrecognized cause of core burstfailure, and on new modifications of the paperboard cores to counteractthe newly identified problem.

[0011] In particular, the inventors have found that chucks used duringwinding operations significantly deform the exterior diameter and shapeof the paperboard core. The deformation is not apparent, however,because after winding is complete and the chucks are disengaged, thecore will normally return to its original size and shape. Because thediameter and shape of the core is deformed during the winding operation,the compressive and friction stresses built into the paper roll arebased on the distorted shape of the core during the winding operation.However, when the chucks are disengaged after winding and the corereturns to its original size and shape, a significant portion of thebeneficial effects of the compressive and friction stresses built intothe paper roll during winding can be lost. The distortion of thepaperboard core and the paper roll build during winding, and the relatedchanges to the paper roll structure upon removal of the core from thewinding chucks, are complicated by further subsequent variable stressesthat can be applied to the paper roll by the unwinding chucks insertedinto the ends of the paper mill core during the unwinding operation. Itis believed that the distortions of the paperboard core caused by thepressure of the unwinding chucks during the unwinding operation can, insome cases, aggravate harmful effects of the distorted stressesgenerated during winding and/or aggravate the loss of beneficialstresses upon removal of the core from the winding chucks, to therebyincrease the possibility of core burst failure. However, in other cases,forces applied by the unwinding chucks to the core ends may counteract,at least in part, the loss of beneficial stresses and/or the distortedwinding stresses built into the paper roll by the distorted core duringthe winding process. It is believed that the dependency of core burstfailure on these separate events associated with winding and unwindinghas further interfered with identification of possible causes of centerburst failure.

[0012] In accordance with the present invention, paperboard cores forpaper mill winding of wide and heavy paper rolls, are modified tosignificantly decrease or minimize outward transmission of forcesapplied to the inside of the core by winding and unwinding chucks. Inparticular, the paper mill cores of the invention are structured toallow winding and unwinding chucks to distort the interior diameter andshape of the interior body wall of the core while resisting distortionof the core exterior, i.e., the corresponding distortion of the coreexterior is significantly less or substantially minimized. Themodifications of paper mill cores according to the invention can beachieved while maintaining high flat crush and dynamic strengthproperties as are required for paper mill cores.

[0013] Paperboard core structures for counteracting center burst failureprovided according to the invention include a multi-ply zone of highstrength, but relatively compliant, paperboard plies within the outer70% (based on the total body wall thickness) of the body wall. The zoneof relatively compliant, high strength paperboard plies has a thicknessof at least about 4 mm. In addition, the overall wall thickness of thecore is preferably at least about 15 mm, and is thus preferablyincreased, as compared to the wall thickness of conventional highstrength wide paper mill cores formed entirely of extremely highstrength, extremely high density, non-compliant paperboard plies.

[0014] Advantageously the relatively compliant, high strength paperboardplies have a density of between about 0.65 and about 0.75 g/cc, morepreferably, between about 0.67 and about 0.73 g/cc. In the case of the 6in. inside diameter core (including both the 152 mm. U.S., and the 150mm. European versions), the zone of relatively compliant, high strengthpaperboard plies is preferably positioned in the central portion of thepaperboard wall. In the case of cores having an inside diameter of about3 in. (76 mm.), the zone of relatively compliant, high strengthpaperboard plies is preferably positioned in the outer 50% of the bodywall, and it is currently preferred that this zone form the exterior 40%of the body wall.

[0015] The improved core structures according to the inventionsubstantially reduce outside diameter changes caused by winding chucks,and also decrease transfer of harmful forces to the paper roll byunwinding chucks. In accord with the invention, this is achieved byreliance on a zone of somewhat more compliant paperboard plies ratherthan by further strengthening the wall of the core for reasons explainedin greater detail subsequently. The preferred increased core wallthickness effectively compliments the zone of more compliant, highstrength paperboard plies to maintain the overall flat crush strengthand dynamic strength of the paperboard core. Moreover, preferredincreases in wall thickness can also increase the maximum allowablewinding speed during winding and unwinding operations (known as criticalspeed).

[0016] In greatly preferred embodiments of the invention, the 6 in. (150or 152 mm.) inside diameter paper mill core has a total wall thicknessof above about 15 mm., more preferably about 16 mm., and thus has anincreased wall thickness as compared to the wall thickness of 13 mm.conventionally used in wide, high strength 6 in. ID cores.Advantageously, the interior 25-40% of the body wall thickness is formedfrom extremely dense, high strength paperboard plies in which thedensity ranges from about 0.80 to about 0.92 g/cc. The central 30-35% ofthe body wall thickness is preferably formed of high strength, morecompliant paperboard plies having a density between about 0.65 and 0.75g/cc. The exterior 30-35% of the core wall is preferably formed of theextremely dense, high strength paperboard used to form the interior zoneof the body wall, as discussed above.

[0017] In the case of a 3 in. (76 mm), inside diameter paperboard coreaccording to the invention, it is preferred that the interior 55-65% ofthe core body wall thickness be formed from the extremely dense, highstrength paperboard plies, set forth above, and that the exterior 35-45%of the wall thickness of the 3 in. (76 mm), inside diameter core beformed from the high strength, more compliant paperboard plies discussedabove. The 3 in. (76 mm.), ID cores preferably have a wall thickness ofabout 17-19 mm as compared to the conventional 15 mm wall thickness usedin 3 in. (76 mm), ID paper mill cores.

[0018] According to yet another embodiment of the invention, a portionof the high strength, more compliant paperboard plies can be mixed withthe extremely high strength paperboard plies in the interior 30% thecore body wall. Nevertheless, in this embodiment, sufficient extremelyhigh strength paperboard plies are provided in the interior 30% of thecore body wall such that at least half of the plies in this portion ofthe body wall are extremely high strength plies; and all, orsubstantially all of the plies forming the interior 15% of the body wallare extremely high strength plies. The total body wall thickness exceedsabout 15 mm; the total thickness of plies in the lower density, highstrength but compliant range exceeds about 5 mm; and the total thicknessof plies in the extremely high density range exceeds about 9 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In the drawings which form a portion of the original disclosureof the invention;

[0020]FIG. 1 schematically illustrates in partial perspective view awinding chuck, shown in cross section, inserted into the end of aconventional paper mill core supporting a portion of a paper roll, withthe lugs of the winding chuck being shown distorting the outsidediameter and shape of the paperboard core in exaggerated detail;

[0021]FIG. 2 schematically illustrates in partial perspective view anunwinding chuck (show in cross-sectional view) of differentconfiguration as compared to the winding chuck shown in FIG. 1, insertedinto the end of a paperboard core and supporting a portion of a paperroll, with the lugs of the unwinding chuck shown engaging the interiorof the paperboard core in a configuration correspondingly different thanthe engagement of the winding chuck of FIG. 1 with the core;

[0022]FIG. 3 illustrates a perspective schematic view of a conventionalpaperboard paper mill core and illustrates the varying distortion of theexterior diameter and shape of the core in three different zones alongthe length of the core as a result of the force applied by aconventional winding apparatus;

[0023]FIG. 4 is a partially broken away cross-sectional viewschematically illustrating one preferred paperboard core according tothe invention;

[0024]FIG. 5 is a partial cross-sectional view illustrating interactionbetween the preferred core shown in FIG. 4 and the lugs of a winding orunwinding chuck;

[0025]FIG. 6 is a partially broken away cross-sectional viewillustrating another preferred paper mill paperboard core according tothe invention; and

[0026]FIG. 7 is a partially broken away cross-sectional viewillustrating yet another advantageous paper mill paperboard coreaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0028]FIG. 1 illustrates a conventional paper mill paper core windingprocess wherein a portion of a paper roll 10 is shown supported on aconventional paperboard core 12, which in turn is supported within itsinterior end portions by a winding chuck 14 which includes a pluralityof radially protruding lugs 16. As is well known to those skilled in theart, the lugs 16 can be moved radially inwardly and radially outwardlyby various well known mechanisms (not shown) in order to allow mountingof the paperboard core 12 on the chuck 14 and engagement of thepaperboard core 12 by the chuck 14.

[0029] In particular, when the lugs 16 are in a radially retractedposition (not shown) the paperboard core can be coaxially mounted on thechuck 14. Retraction of the lugs 16 at the completion of a windingoperation also allows removal of the paperboard core 12 and the fullpaper roll 10 from the chuck 14. On the other hand, when the lugs 16 areextended radially outwardly, the lugs forcefully contact the insidesurface of the paperboard core 12 so that rotation of the chuck 14 alsorotates the core 12.

[0030] The conventional chucks 14 extend axially only partially into theends of the core 12, typically for a length of between about 2 inches(51 mm) and about 5 inches (127 mm). Because of the extremely highweight of the paper roll 10, (i.e., exceeding two tons (4000 lbs., 1,800kg), more typically approaching or exceeding five tons (10,000 lbs.,4,500 kg.)), and also because of its substantial length, (typicallybetween about 100 in. (2.5 m) and 142 in. (3.6 m)), and further becauseof the substantial torque that must be transmitted from the chuck 14 tothe conventional paperboard core 12, the lugs 16 of the chuck 14 aredesigned to expand radially outwardly. The lugs 16 are driven outwardlyby pressure from a torque activation mechanism that forces the lugs 16outwardly a distance sufficient to engage the interior surface of thecore

[0031] The conventional paperboard core 12 used for paper mill windingof wide, heavy paper rolls is formed entirely of a plurality of plies ofextremely high density, extremely high strength paperboard. Typically,the conventional 6 in. ID core is formed from about 20-25 plies ofextremely high density and high strength paperboard, that is, paperboardhaving a density typically greater than about 0.80 g/cc, ranging up toabout 0.92 g/cc.

[0032] It was recognized by the present inventors that the extremelyhigh density of the paperboard plies that are used to form theconventional core for wide, heavy paper rolls, is of such a high densitythat absorption of the chuck forces applied to the conventionalpaperboard core 12, by compression of the wall of paperboard core 12, istheoretically limited. Accordingly, the inventors designed experimentsapplying a plurality of miniature strain gauges to the surface of aconventional winding core in order to measure the effects of theconventional winding chuck 14, such as that illustrated in FIG. 1, to aconventional paperboard core 12 during a real time winding operation. Asa result of these tests, it was found that the exterior of thepaperboard cores were significantly distorted during the windingprocess. The distortion of the paperboard core at its ends isillustrated in FIG. 1. In particular, the lugs 16 interact with thecorresponding overlying portion of the conventional paperboard coreillustrated in the area 20 shown in FIG. 1 such that these portions ofthe core are bent outwardly causing the shape of the core to evolvetowards the shape of a square (or other polygon) as illustrated inFIG. 1. Thus, because of the extremely high density of the paperboardcore, distortion of the interior shape of the core by the lugs 12produces a corresponding distortion of the exterior of the core 12. Inturn, the areas 22 of the paper roll 10 that overlie the extended areas20 of the paperboard core 12 are forced outwardly to a greater extentthan the areas 24 of the roll 10 which do not overlie a lug 16 of thechuck 14.

[0033] It was also found that when the winding operation has beencompleted and when the fully wound roll of paper 10 supported on theconventional winding core 12, is removed from the chuck 14, thesignificant circumferential strains observed on the surface of thepaperboard core 12 are relieved. As a practical matter, this means thatthe paperboard core 12 returns to its original circular shape. Thus, theareas 22 of the paper roll 10 wound on top of the extended portions 20of the paperboard core 12 can lose the beneficial effects of radial andfriction stresses developed during the winding process, that in turn canhelp maintain roll integrity until the unwind operation. In addition, itis believed that in some cases the areas 22 of the paper roll 10, canalso retain harmful effects as a result of differential pressure,corresponding to the distorted outside shape of the paperboard core,during winding.

[0034] With reference to FIG. 2, when the roll of paper 10 is used bythe end user, i.e. by the printer, the previously distorted paperboardcore 12 is supported within its interior ends by an unwinding chuck 34.However, in many cases, the design and construction of the unwindingchucks 34 is completely different than the construction of the windingchucks 14. This is shown for the purposes of illustration only by theillustration of three lugs 36 in the unwinding chuck 34 as opposed tothe four lugs 16 in the winding chuck 14.

[0035] During unwinding, expansion of the lugs 36 of the unwinding chuck34 applies substantial radial force to the ID and therefore the OD ofthe previously distorted paperboard core 12, as indicated by force lines38. Because of differences between the winding and unwinding chuckstructures, or between the placement of the roll ends with respect tothe chuck lugs during the different winding and unwinding operations,the outside of the core at the ends can transmit a substantiallydifferent pattern of forces to the paper roll during unwinding,(illustrated in FIG. 2 as triangularly distributed forces) as comparedto the forces applied to the paper roll by the distorted core duringwinding.

[0036] In some cases, the portion of the paperboard core 20 that wasdistorted outwardly during the winding operation illustrated in FIG. 1,can be positioned during unwinding at a location, identified as location40 in FIG. 2, between two extended lugs 36 and no longer overlying theextended lugs as was the case during winding, i.e., overlying lugs 16shown in FIG. 1. Accordingly, significant radial pressure relief canoccur in such a situation between the core and the paper in the roll 10in the areas 42 overlying the previously extended portion of thepaperboard core. Similarly, areas 44 of the paper roll 10 which overliethe extended lugs 36 of the unwinding chuck 34, can correspond to areas24 of the paper roll shown in FIG. 1, that were positioned between, andnot above, the extended lugs 16 of the winding chuck 14. This can resultin application of strains to the paper roll that are distributedcircumferentially in a significantly different pattern, as compared towinding strains, to thereby aggravate any harmful effects retained inthe paper roll as a result of the distorted stresses applied duringwinding. On the other hand, if the lugs 36 of the unwinding chuck 34 arealigned with the paper roll in a manner that substantially correspondsto the original alignment of the lugs 16 of the winding chuck 14 duringthe winding operation, the stresses applied to the paper roll by theunwinding chuck 34 can potentially mitigate damage resulting from theloss of beneficial radial and friction winding stresses that occurredupon disengagement of the winding chucks (as discussed earlier).

[0037] The dynamic stresses applied to the paper roll at its ends duringwinding imply the presence of dynamic shear strain (axial sliding of thesheet forming the paper roll between the layers) at the ends of theroll. Moreover, because the dynamic stresses are substantially reducedafter 100-200 mm of paper are outwardly wound on the paper roll, thedynamic shear strain is generally present only at the interior portionsof the paper roll, i.e., the portions of the paper roll in which thepaper is damaged in center burst failure of a paper roll during aprinting operation.

[0038] Turning now to FIG. 3, the distorted shape of the paperboard coreduring winding is illustrated. As seen in FIG. 3, the wide paperboardcore 10 includes three zones along its length including two end portions50 and a longer middle portion 52. As generally illustrated in FIG. 3,the circumferential deformation of the core occurs generally in the endportions 50 of the core whereas the middle portion 52 of the core is notdistorted because the chuck 14 as shown in FIG. 1 extends only into theend portions 50 of the core as illustrated in FIG. 3. In addition, asillustrated generally in FIG. 3, the portions of the core 20 thatoverlie an extended lug 16 of the winding chuck 14 shown in FIG. 1, areextended outwardly more than the areas 24 of the end portions 50 of thecore which are located between the areas 20. Accordingly, the complexityof the compressional and frictional forces applied during winding to theportion of the paper roll near the core will be apparent from thedistorted core shape generally illustrated in FIG. 3.

[0039]FIG. 4 illustrates a preferred core structure in accordance withthe present invention. The core structure of the invention 100illustrated in FIG. 4 is currently preferred structure for 6 in. (150 or152 mm) ID paper mill cores of the invention. As illustrated in FIG. 4,the wall of the core 100 includes three multilayer zones 102, 104, and106 positioned sequentially from the interior portion of the wall ofcore 100 to the exterior portion of the wall of core 100. Each of thethree zones 102, 104, and 106 include a plurality of paperboard plies102 a, 104 a, and 106 a respectively. The plies 102 a in the zone 102are formed of extremely high density, extremely high strengthpaperboard, that is, paperboard having a density exceeding about 0.80g/cc, advantageously between about 0.80 g/cc and about 0.92 g/cc, andpreferably have a density of about 0.82 g/cc or higher, and mostpreferably a density in the range of between about 0.82 and about 0.90g/cc.

[0040] Paperboard densities are determined for the purposes of thesubject invention in accordance with the TAPPI 220 and 411 standardtests. According to these tests, the paperboard is fully conditioned at73° plus or minus one degree F. and at 50% plus or minus 2% relativehumidity until it reaches equilibrium. Thereafter at least five samplesof paperboard are measured for thickness and area and are weighed.Density is then determined by dividing the weight in grams by the volumein cubic centimeters.

[0041] Returning now to FIG. 4, the multiple plies 106 a forming zone orlayer 106 of the wall of core 100 as shown in FIG. 4, are alsoadvantageously formed of extremely high density, extremely high strengthpaperboard as discussed above in connection with zone 102 of FIG. 4. Themultiple plies 104 a forming the central or middle zone 104 of the wallof paperboard core 100 are formed from a high strength but relativelycompliant paperboard, i.e., paperboard having a density of between about0.65 and about 0.75 g/cc, more preferably between about 0.67 and about0.73 g/cc. The outermost ply or plies 110, which is optional, can be aply or several plies (i.e., one to three plies, typically one or twoplies) that are different from the extremely high density, extremelyhigh strength paperboard plies of zone 102, and also different from thehigh strength but relatively compliant paperboard plies of zone 104. Inthis regard, is to be noted that outer plies of winding cores are oftenchosen in order to impart various surface friction or decorative aspectsto the exterior of the core body; and/or to improve the manufacturingprocess, e.g., a spiral winding or linear draw process; and/or toimprove adhesion of the exterior ply; as will be apparent to those ofskill in the art. Similarly, the interior-most ply or plies 111 of thecore body can be varied in the manner and for the reasons set forthabove in regard to the ply or plies 110.

[0042] For reasons discussed below in connection with FIG. 5, zone 104of the wall of the core illustrated in FIG. 4, has a thickness 114preferably of at least about 4 mm. Even more preferably, zone 104 has athickness 114 greater than about 4.5 mm, preferably greater than about 5mm, more preferably between about 4.5 and 6.5 mm. Currently, it ispreferred that zone 104 have a thickness of between about 5 and about 6mm, most preferably about 5.6 mm. It is also preferred that the zone104, formed of the compliant, high strength paperboard plies, constitutebetween about 25 and 40% of the total wall thickness of the corepreferably between about 30 and 35% of the wall thickness.

[0043] The zones 102 and 106 formed of the extremely high density,extremely high strength paperboard plies, each have a thickness, 116 and118, respectively, constituting between about 25 and about 40% of thetotal wall thickness of the core 100. Preferably, each of zones 102 and106 constitutes between about 30 and 35% of the total wall thickness ofthe core 100. It is currently preferred that each of the zones 102 and106 have a thickness, 116 and 118, respectively, constituting about 33%of the total wall thickness of the core 100.

[0044]FIG. 5 illustrates in exaggerated detail how the core structuresof the invention interact with the radially expanding lugs 16 of aconventional chuck 14. In particular, as seen in FIG. 5, radial outwardexpansion of the lugs 16 causes the intermost zone 102 formed fromextremely high density, extremely high strength paperboard plies todeform out a cylindrical configuration. Thus, each of the portions 120 aof zone 102 of the core body wall are pushed outwardly to form “cornersor verticies” of a square-like or polygon-like shape. However, becausethe multiple plies 104 a of zone 104 are a relatively compliant and lessdense paperboard material, the portions 120 b of the zone 104 whichoverlie the lugs 16, are capable of absorbing all or a substantialportion of the radial outward expansion of the portion 120 a of theinterior zone 102 of the body wall of the core 100. The portion 120 c ofthe exterior zone 106 of the wall of the core 100 is preferably expandedradially outwardly to only a minimal extent because of the strain energyabsorption by zone 104.

[0045] In general, and particularly in the construction of 6 inchinterior diameter paperboard cores such as illustrated in FIGS. 4 and 5,it is preferred that a zone of extremely high strength, extremely highdensity paperboard, i.e., zone 106 in FIG. 4, be positioned outwardly ofthe relatively compliant, high strength paperboard zone 104. This isbelieved to enhance the absorption by the zone 104 of the radialexpansion effected by the lugs 16, without causing substantial exteriordistortion of the core body 100.

[0046] Because of the extremely high density of the paperboard pliesforming the interior and exterior zones 102 and 106, respectively, ofthe body wall 100 shown in FIGS. 4 and 5, the radial outward expansionof the lug 16 of the chuck 14 (as illustrated in FIG. 5) does notsignificantly compress the thickness 116 of the interior zone 102, orthe thickness 118 of the exterior zone 106 of the body wall. However,because zone 104 is formed of a relatively compliant, lower densitypaperboard, the thickness 114 of zone 104 can compress, particularly inthose portions 120 b of zone 104 which overlie the outwardly expandinglugs 16 of the chuck 14. In general, the total thickness and number ofplies in the relatively compliant zone 104 are selected to allowabsorption of the expansion distance of the chuck lugs 16. Thus, wherelower density paperboard materials within the lower portion of thepreferred range are selected for the formation of the zone 104, thetotal thickness of zone 104 can be less as compared to the situationwhere higher density paperboard materials within the preferred range areselected for the formation of the zone 104.

[0047] In a preferred embodiment, the total number of plies used to formthe body wall of core 100 as shown in FIGS. 4 and 5, will range fromabout 25 to about 35 plies, preferably from about 28 to about 32 plies.In general, the higher density plies will typically have a smallerthickness as compared to the lower density plies. For example, the bodywall of the core 100 can advantageously be formed of a plurality ofextremely high density, extremely high strength paperboard plies eachhaving a total thickness of about 0.022 inch (0.56 mm), and a pluralityof relatively compliant, high strength paperboard plies having athickness of about 0.025 inch (0.64 mm).

[0048] As will be well understood by those of ordinary skill in the art,the thickness and density of paperboard plies can be widely varied.Preferably, the paperboard plies used in the invention will each have adensity between about 0.65 and about 0.92 g/cc, more preferably betweenabout 0.67 and about 0.90 g/cc. Paperboard strength and density aregenerally varied by varying pulp treatments, by varying the degree ofnip compression and by variations in the raw materials forming the pulp.Paperboard densities and strengths can also be changed by employingvarious known additives and strengthening agents during the papermakingprocesses. Paperboard plies useful herein will typically have athickness within the range of between about 0.020 inch (0.51 mm) andabout 0.035 inch (0.89 mm), more typically between about 0.022 inch(0.56 mm) and about 0.030 inch (0.76 mm).

[0049] In general, the present invention addresses a number ofpreviously unrecognized problems and causation factors associated withthe center burst problem. Significantly, the strength of extremely highdensity, extremely high strength paperboard plies such as are normallyused to form the body wall of wide paper mill cores is derived in largepart from a high pressure compression of high quality paper pulp.However, the resultant high density, high strength paperboard retainsextremely little capacity for further compression, i.e., for furtherreduction in thickness. Accordingly, the present invention relies uponthe use of a plurality of relatively strong paperboard plies, each stillretaining the capacity of further compression and thickness reduction.Therefore, the lower density, high strength paperboard plies employed inthe present invention are capable of absorbing a substantial amount ofthe radial expansion of the lugs of a conventional chuck to therebycounteract the radial expansion that would otherwise be transmitted in acorresponding amount to the exterior of the paperboard core.

[0050]FIG. 6 illustrates a preferred core structure of the invention asapplied to a 3 inch (76 mm) inside diameter paper mill core. The coreillustrated in FIG. 6 includes two zones, an interior zone 202 and anexterior zone 204. The interior zone 202 is formed of a plurality ofextremely high density, extremely high strength paperboard plies 202 awhile the exterior zone 204 is formed from a plurality of high strength,but relatively compliant plies 204 a. In the core structure illustratedin FIG. 6, the zone of high strength but relatively compliant paperboardlayers, 204, is advantageously positioned in the exterior 50% of thebody wall of the paperboard core. Preferably, the zone 204 has athickness 214, which constitutes about 30 to about 45% of the body wallthickness, most preferably between about 35 and 45% of the wallthickness, e.g., about 40% of the wall thickness. Similarly the interiorzone 202, has a thickness 216, which constitutes about 50% to about 70%of the body wall thickness, most preferably between about 55% and 65% ofthe wall thickness, e.g., about 60% of the wall thickness.

[0051] When the high strength, relatively compliant density paperboardplies are positioned in a zone on or near the exterior of the body wall,like in the structure illustrated in FIG. 6, it is preferred that thetotal thickness 214 of the zone be at least about 5 mm, preferably fromabout 6 to about 9 mm. A preferred thickness 214 for the zone 204 of the3 inch (76 mm) ID paperboard core structure illustrated in FIG. 6 isfrom about 6.8 to about 7.2 mm. The currently preferred total wallthickness of the (3 inch (76 mm) ID paperboard core illustrated in FIG.6 is from about 17 to about 19 mm.

[0052] In addition, the core 200 of FIG. 6 can optionally include anoutermost ply or plies 210, that are different from the extremely highdensity, extremely high strength paperboard plies of zone 202, and alsodifferent from the high strength but relatively compliant paperboardplies of zone 204. The outer ply or plies 210, when present, are variedin the manner and for the reasons set forth above in connection with theply or plies 110 of FIG. 4, as will be apparent to those of skill in theart. Similarly, the interior-most ply or plies 211 of the core body 200of FIG. 6 can be varied in the manner and for the reasons.

[0053] Yet another embodiment of the invention is illustrated in FIG. 7.The paper mill core of FIG. 7 is advantageously a 6 inch (150 mm or 152mm) ID paper mill core. The core illustrated in FIG. 7 includes fivezones, an interior zone 302, an exterior zone 312, a central zone 306,and two zones 304 and 308, positioned, respectively, between theinterior and central zones, and between the exterior and central zones.In paper mill core structure of FIG. 7, the interior zone 302 is formedof a plurality of extremely high density, extremely high strengthpaperboard plies 302 a while the exterior zone 312 is also formed of aplurality of extremely high strength paperboard plies 312 a. Similarly,the central zone 306 is formed of a plurality of extremely highstrength, extremely high density paperboard plies. The zones 304 and 308are each formed of a plurality of lower density, relatively complianthigh strength paperboard plies 304 a and 308 a, respectively.

[0054] In the paperboard core structure illustrated in FIG. 7, theinterior and exterior extremely high strength, high density paperboardzones 302 and 312, respectively, each constitute about one-sixth of thetotal thickness of the wall structure. Likewise, each of the paperboardzones 304 and 308 formed of lower density, relatively compliant highstrength paperboard plies, constitute about one-sixth of the totalthickness of the paperboard body wall. The central zone 306, formed ofextremely high strength, extremely high density paperboard pliespreferably has a thickness of about one-third of the thickness of thebody wall. The body wall preferably has a total thickness of at leastabout 15 mm. The core 300 of FIG. 7 can optionally include an outermostply or plies 310, that are different from the extremely high density,extremely high strength paperboard plies of zones 302, 312, and 306, andalso different from the high strength but relatively compliantpaperboard plies of zones 304 and 308. The outer ply or plies 310, whenpresent, are varied in the manner and for the reasons set forth above inconnection with the outer ply or plies of FIGS. 4 and 6, as will beapparent to those of skill in the art. Similarly, the interior-most plyor plies 311 of the core body 300 of FIG. 7 can be varied in the mannerand for the reasons.

[0055] The paperboard core structure illustrated in FIG. 7 is currentlynot preferred; however, it does provide considerable strength propertieswhile substantially decreasing distortion of the core exterior upon theapplication of significant radial force to the core interior.

[0056] The wide high strength paperboard cores of the invention ingeneral have a length greater than about 100 inches, typically greaterthan about 120 inches (3 m) more typically about 142 inches (3.6 m) orgreater. Advantageously, the paperboard cores have a minimal body wallthickness of about 13 mm and preferably have a total body wall thicknessof at least about 15 mm or greater. Most preferably, the total body wallthickness will exceed the conventional body wall thickness used for ahigh strength paperboard core of corresponding ID. Thus, the body wallthickness of the 6 inch (152 mm) ID core illustrated in FIGS. 4 and 5will preferably exceed about 15 mm., thus exceeding the standard 13 mmbody wall thickness. Similarly, the body wall thickness of the 3 inch(76 mm.) ID core illustrated in FIG. 6 will preferably exceed theconventional 15 mm body wall thickness for 3 inch (76 mm.), ID cores.

[0057] The use of a total body wall thickness exceeding the body wallthickness of a conventional wide, high strength paperboard core of thesame or comparable ID provides several significant benefits andadvantages. In particular, increasing the body wall thicknesscompensates for the use of a smaller amount (i.e., a smaller number ofplies) of extremely high density, extremely high strength paperboardplies, as compared to the conventional structures, while still providinga flat crush strength that is comparable to or exceeds the flat crushstrength of the conventional structures. In general, the paperboardcores of the present invention will have an extremely high flat crushstrength of at least about 200 lbs/in (3500 N/100 mm). For example, aconventional 6 inch ID high strength paperboard core formed entirely ofextremely high density, extremely high strength paperboard plies has aflat crush strength of about 200 lbs/in (3500 N/100 mm). A preferred 6inch ID paperboard core as illustrated in FIGS. 4 and 5 can readily havea total wall thickness of 16 mm (0.630 inches) and is thus thicker thanthe conventional structure in an amount of about 3 mm, thus representinga 23% increase in wall thickness. However, only about 65-70% of thetotal wall thickness in this case, is formed of extremely high strength,extremely high density paperboard; thus, the structure illustrated inFIGS. 4 and 5 is preferably formed from only about 80% of the extremelyhigh strength, extremely high density paperboard as would be used toform the conventional structure. Nevertheless, the preferred paperboardcore structure illustrated in FIGS. 4 and 5 can readily have a flatcrush strength of about 220 lbs/in (3850 N/100 mm). In addition, thepreferred structure illustrated in FIGS. 4 and 5 reduces the amount thecore OD expansion for chuck engagement by about 30% compared to aconventional core.

[0058] Similarly, the preferred structures for 3 inch (76 mm.), insidediameter cores as illustrated in FIG. 6 can readily have a 25% coreburst improvement (reduction in the amount the core OD expansion forchuck engagement) as compared to the conventional 15 mm wall thicknessstructure. Nevertheless, the flat crush strength of the preferredstructures illustrated in FIG. 6 can readily be about 312 lbs/in (5425N/100 mm) as compared to the conventional flat crush strength for a 15mm body wall thickness 3 inch (76 mm.), ID core, of about 300lbs/in(5250 N/100 mm). In addition, the dynamic strength properties ofthe paperboard core illustrated in FIG. 6 preferably is comparable to orexceeds the dynamic strength of the conventional comparable ID core.

[0059] The use of a higher wall thickness in accord with the presentinvention also enables attainment of a higher allowable rotationalspeed, or “critical speed”. Accordingly, the preferred paperboard coresaccording to the invention can be rotated at speeds from about 3 toabout 5% greater than the critical rotation speed of conventional corestructures. Preferably the increase in rotational speed is achieved byuse of a total body wall thickness greater than about 15 mm in eitherembodiment of the invention The greater wall thickness associated withthe preferred structures increases core OD. The increased OD results ina lower rotational speed for any web speed during winding and unwinding.Thus, the critical speed performance is improved.

[0060] Exemplary currently preferred paper mill winding cores accordingto the invention have the constructions set forth below. Construction 1ID: 150.4 mm OD: 182.4 mm Wall Thickness (estimated): 16 mm Ply Density(g/cc) Thickness (micron) Inside* about 600 0.78 Plies 2-10 0.9  550Plies 11-18 0.72 620 Plies 19-27 0.9  550 Ply 28** 0.68 740 Outer Ply NA220

[0061] Construction 2 ID: 78.7 mm OD: 110.7 mm Wall Thickness(estimated): 17 mm Ply Density (g/cc) Thickness (micron) Inside* about620 0.72 Plies 2-18 0.9  550 Plies 19-29 0.72 620 Ply 30** 0.68 740Outer Ply NA 220

[0062] Construction 3 ID: 3 inch OD (estimated): 3.72 inch WallThickness (estimated): 0.717 inch Ply Density (g/cc) Thickness (inch)Inside* 0.76 0.025 Plies 2-21 0.82 0.022 Plies 22-32 0.68 0.025 OuterPly** NA 0.013

[0063] Construction 4 ID: 6 inch OD (estimated): 6.63 inch WallThickness (estimated): 0.633 inch Ply Density (g/cc) Thickness (inch)Inside* 0.76 0.025 Plies 2-11 0.9  0.022 Plies 12-31 0.76 0.025 Plies32-37 0.9  0.022 Ply 38** 0.76 0.025 Outer Ply** NA 0.013

[0064] Construction 5 ID: 6 inch OD (estimated): 6.63 inch WallThickness (estimated): 0.633 inch Ply Density (g/cc) Thickness (inch)Inside* 0.76 0.025 Plies 2-11 0.82 0.022 Plies 12-31 0.68 0.025 Plies32-37 0.82 0.022 Ply 38** 0.76 0.025 Outer Ply** 0.65 0.013

[0065] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A multi-ply paperboard core for supporting aroll of wide, continuous paper mill sheet comprising: a multi-plypaperboard core structure having a length exceeding about 100 in. (255cm.), and being defined by a generally cylindrical body wall having athickness of at least about 15 mm; said body wall including a radiallyinterior multi-ply zone constituting at least about 25% of the totalthickness of the body wall and consisting essentially of paperboardplies, each having a density exceeding about 0.80 g/cc; and, a secondmulti-ply zone within the radially outer 70% of the total thickness ofthe body wall consisting essentially of paperboard plies each having adensity of between about 0.65 and about 0.75 g/cc, said second zonehaving a thickness of at least about
 4. 2. The multi-ply paperboard corefor supporting a roll of wide continuous paper mill sheet according toclaim 1 wherein said paperboard plies in said interior multi-ply zoneeach have a density between about 0.82 and 0.90 g/cc.
 3. The multi-plypaperboard core for supporting a roll of wide continuous paper millsheet according to claim 1 wherein said paperboard plies in said secondmulti-ply zone each have a density of between about 0.67 and 0.73 g/cc.4. The multi-ply paperboard core for supporting a roll of widecontinuous paper mill sheet according to claim 2 wherein said paperboardplies in said second multi-ply zone each have a density of between about0.67 and 0.73 g/cc.
 5. The multi-ply paperboard core for supporting aroll of wide continuous paper mill sheet according to claim 1 whereinsaid interior multi-ply zone constitutes between about 30 and 35% of thetotal thickness of the body wall.
 6. The multi-ply paperboard core forsupporting a roll of wide continuous paper mill sheet according to claim5 wherein said second multi-ply zone has a thickness of between about 30and 35% of the total thickness of the body wall.
 7. The multi-plypaperboard core for supporting a roll of wide continuous paper millsheet according to claim 1 wherein said second multi-ply zone has athickness of between about 40 and about 50% of the total thickness ofthe body wall.
 8. A multi-ply paperboard core for supporting a roll ofwide, continuous paper mill sheet comprising: a multi-ply paperboardcore structure having a length exceeding about 100 in. (255 cm.), andbeing defined by a generally cylindrical body wall having a thickness ofat least about 15 mm, and an inside diameter of about 6 in. (152 mm);said body wall including a radially interior multi-ply zone constitutingabout 25% to about 35% of the total thickness of the body wall andconsisting essentially of paperboard plies, each having a densityexceeding about 0.80 g/cc; a second multi-ply zone within a centralportion of the body wall consisting essentially of paperboard plies eachhaving a density between about 0.65 and 0.75 g/cc., said second zoneconstituting between about 35% and about 45% of the total thickness ofthe body wall, said second zone having a thickness of at least about 4mm.; and a third multi-ply zone within a radially outer portion of thebody wall consisting essentially of paperboard plies each having adensity exceeding about 0.80 g/cc., said third zone constituting betweenabout 30% and about 35% of the total thickness of the body wall.
 9. Themulti-ply paperboard core for supporting a roll of wide continuous papermill sheet according to claim 8 wherein said paperboard plies in saidradially interior multi-ply zone each have a density between about 0.82and 0.90 g/cc.
 10. The multi-ply paperboard core for supporting a rollof wide continuous paper mill sheet according to claim 9 wherein saidpaperboard plies in said second multi-ply zone each have a density ofbetween about 0.67 and 0.73 g/cc.
 11. The multi-ply paperboard core forsupporting a roll of wide continuous paper mill sheet according to claim8 wherein said paperboard plies in said second multi-ply zone each havea density of between about 0.67 and 0.73 g/cc.
 12. The multi-plypaperboard core for supporting a roll of wide continuous paper millsheet according to claim 9 wherein said interior multi-ply zoneconstitutes about 33% of the total thickness of the body wall.
 13. Themulti-ply paperboard core for supporting a roll of wide continuous papermill sheet according to claim 9 wherein said second multi-ply zone has athickness of about 33% of the total thickness of the body wall.
 14. Amulti-ply paperboard core for supporting a roll of wide, continuouspaper mill sheet comprising: a multi-ply paperboard core structurehaving a length exceeding about 100 in. (255 cm.), and being defined bya generally cylindrical body wall having a thickness of at least about15 mm, and an inside diameter of about 3 in. (76.2 mm); said body wallincluding a radially interior multi-ply zone constituting at least about50% of the total thickness of the body wall and consisting essentiallyof paperboard plies each having a density exceeding about 0.80 g/cc; anda second multi-ply zone within the radially outer 50% of the body wallthickness consisting essentially of paperboard plies each having adensity between about 0.65 and 0.75 g/cc., said second zone having athickness of at least about 5 mm.
 15. The multi-ply paperboard core forsupporting a roll of wide continuous paper mill sheet according to claim14, wherein said paperboard plies in said radially interior multi-plyzone each have a density between about 0.82 and 0.90 g/cc.
 16. Themulti-ply paperboard core for supporting a roll of wide continuous papermill sheet according to claim 14, wherein said paperboard plies in saidsecond multi-ply zone each have a density of between about 0.67 and 0.73g/cc.
 17. The multi-ply paperboard core for supporting a roll of widecontinuous paper mill sheet according to claim 15, wherein saidpaperboard plies in said second multi-ply zone each have a density ofbetween about 0.67 and 0.73 g/cc.
 18. The multi-ply paperboard core forsupporting a roll of wide continuous paper mill sheet according to claim15, wherein said interior multi-ply zone constitutes about 55 to about65% of the total thickness of the body wall.
 19. The multi-plypaperboard core for supporting a roll of wide continuous paper millsheet according to claim 18, wherein said second multi-ply zone has athickness of between about 35 and about 45% of the total thickness ofthe body wall.
 20. The multi-ply paperboard core for supporting a rollof wide continuous paper mill sheet according to claim 19, wherein saidbody wall has a total thickness of about 16 mm or greater.
 21. Apaperboard core for supporting a roll of wide continuous paper millsheet, the paperboard core being constructed to have enhanced resistanceto center burst, and comprising: a cylindrical body wall formed of aplurality of paperboard plies, the body wall having a thickness of atleast about 15 mm, the body wall having only two multi-ply zones, aradially interior one of the zones consisting essentially of paperboardplies each having a density exceeding about 0.8 g/cc, and a radiallyouter one of the zones consisting essentially of paperboard plies eachhaving a density between about 0.65 g/cc and about 0.75 g/cc, saidradially outer zone lying radially outward of the radially interior zoneand having a thickness of at least about 5 mm.
 22. The paperboard coreof claim 21, wherein the radially outer zone occupies about 35% to about45% of the total thickness of the body wall.
 23. The paperboard core ofclaim 22, wherein the radially interior zone occupies about 55% to about65% of the total thickness of the body wall.
 24. The paperboard core ofclaim 21, wherein the paperboard plies in the radially outer zone eachhas a density between about 0.67 g/cc and about 0.73 g/cc.
 25. Thepaperboard core of claim 24, wherein the paperboard plies in theradially interior zone each has a density between about 0.82 g/cc andabout 0.90 g/cc.
 26. The paperboard core of claim 21, wherein thepaperboard plies in the radially interior zone each has a densitybetween about 0.82 g/cc and about 0.90 g/cc.
 27. The paperboard core ofclaim 21, wherein the body wall has an inside diameter of about 3 inches(76.2 mm).
 28. A paperboard core for supporting a roll of widecontinuous paper mill sheet, the paperboard core being constructed toreduce changes in outside diameter size and shape caused by radiallyexpanding a chuck against an inside diameter of the core, the corecomprising: a cylindrical body wall formed of a plurality of paperboardplies, the body wall having a thickness of at least about 15 mm, theplies consisting essentially of a plurality of relatively less-compliantplies each having a density exceeding about 0.80 g/cc and a plurality ofrelatively more-compliant plies each having a density from about 0.65g/cc to about 0.75 g/cc, the relatively more-compliant plies beingdisposed in the radially outer 50% of the body wall thickness, and therelatively less-compliant plies being located only radially inward ofthe relatively more-compliant plies, whereby radially outward expansionof the inside diameter of the core caused by an expanding chuck is atleast partially absorbed by the relatively more-compliant plies suchthat changes in outside diameter size and shape are reduced.
 29. Thepaperboard core of claim 28, wherein the less-compliant pliescollectively make up from about 55% to about 65% of the total thicknessof the body wall.
 30. The paperboard core of claim 28, wherein themore-compliant plies collectively make up from about 35% to about 45% ofthe total thickness of the body wall.